Add a schema to gatekeeper package

[joshrosso]

What this PR does / why we need it

This adds a schema defintion to the gatekeeper package, which enables
the tanzu CLI to provide a list of configurable values.

Details for the Release Notes (PLEASE PROVIDE)

Schema added to the gatekeeper package. This is now exposed via `tanzu package
available get gatekeeper.community.tanzu.vmware.com/1.0.0 --values-schema`.

Which issue(s) this PR fixes

• Fixes: Add schema/properties to gatekeeper package #1938

Describe testing done for PR

1. Bootstrap a cluster.

   tanzu standalone-cluster create -i docker test

2. Apply the package.

   kubectl apply -f addons/packages/gatekeeper/metadata.yaml
   kubectl apply -f addons/packages/gatekeeper/3.2.3/package.yaml

3. Verify values are now exposed.

   tanzu package available get gatekeeper.community.tanzu.vmware.com/1.0.0 --values-schema
   
   | Retrieving package details for gatekeeper.community.tanzu.vmware.com/1.0.0...
     KEY        DEFAULT            TYPE    DESCRIPTION
     namespace  gatekeeper-system  string  The namespace in which to deploy gatekeeper.

4. Verify the package installs

   tanzu package install gk --package-name gatekeeper.community.tanzu.vmware.com --version 1.0.0

   $ kubectl get po -A
   NAMESPACE           NAME                                                     READY   STATUS    RESTARTS   AGE
   gatekeeper-system   gatekeeper-audit-7fdbcfdb8-jl8ft                         1/1     Running   0          38s
   gatekeeper-system   gatekeeper-controller-manager-f8887b97d-hjxnj            1/1     Running   0          38s
   kube-system         antrea-agent-fq7gs                                       2/2     Running   0          11m
   kube-system         antrea-agent-xftl2                                       2/2     Running   0          11m
   kube-system         antrea-controller-86f8988c5f-tbbqk                       1/1     Running   0          11m
   kube-system         coredns-8dcb5c56b-482kd                                  1/1     Running   0          11m
   kube-system         coredns-8dcb5c56b-zmfm6                                  1/1     Running   0          11m
   kube-system         etcd-henro-control-plane-qq6cn                           1/1     Running   0          11m
   kube-system         kube-apiserver-henro-control-plane-qq6cn                 1/1     Running   0          11m
   kube-system         kube-controller-manager-henro-control-plane-qq6cn        1/1     Running   0          11m
   kube-system         kube-proxy-2lhmf                                         1/1     Running   0          11m
   kube-system         kube-proxy-qvzv7                                         1/1     Running   0          11m
   kube-system         kube-scheduler-henro-control-plane-qq6cn                 1/1     Running   0          11m
   tkg-system          kapp-controller-6499b8866-fhlmv                          1/1     Running   0          11m
   tkg-system          tanzu-capabilities-controller-manager-6ff97656b8-z7dcb   1/1     Running   0          11m
   tkr-system          tkr-controller-manager-6bc455b5d4-w8thd                  1/1     Running   0          11m

Special notes for your reviewer

Alone with reviewing the diff, please validate the testing described above.

[joshrosso]

@jpmcb if you've got time to today to look at that i'd greatly appreciate it!

[seemiller]

LGTM